Monolithic weighing systems as the core element of weighing devices are known to the person skilled in this art from precision balances, in particular, from precision balances that work on the principle of electromagnetic force compensation. Weighing systems of this type have a load holder, which is articulated on a base by a parallel link arrangement. In the resulting precision balance the load holder is connected to a weighing pan that holds samples to be weighed, whereas the base is fixed in position. The load holder is able to perform a strictly vertical motion, at least in the event of small deflections, due to the parallel link arrangement. A lever arrangement for transmitting the deflection of the load holder is articulated on the load holder. The lever arrangement is provided with an attachment point for a force compensating arrangement, for example, an electromagnetic moving coil arrangement. The force compensating arrangement is used to generate a counter force that acts on the lever arrangement and that compensates for the deflection of the lever arrangement, where the deflection is produced by the force of gravity acting on the load holder. In the event that the force compensating arrangement is designed as an electromagnetic moving coil arrangement, it is necessary to control a current through the moving coil in order to compensate for the force. The corresponding control circuit includes an optical position sensor that accurately detects the deflections of the lever arrangement, and the electronic control unit provides a corresponding signal, so that an immediate compensation can take place. With this approach it is possible to achieve the objective that the actual deflections of the lever arrangement can be minimized. To some extent it is referred to as a “path-free” force compensation.
The sensitivity of the position sensor is essential for precision weighing. In commercially available weighing systems the lever arrangement has a slotted diaphragm, which is monitored by an optical sensor, in a section of the lever that is designed as a thin wall and that is subject to a comparatively large deflection, in the plane of deflection. In particular, it has proven to be useful to arrange a slotted diaphragm of a thin walled lever section between a photo emitter and a photo detector, so that the light of the photo emitter transilluminates the slotted diaphragm on its way to the photo detector. In the event that the photo detector is suitably design as, for example, a split photodiode, the deflections of the lever section bearing the slotted diaphragm can be very accurately registered. For this purpose pedestals for mounting the photo emitter and the photo detector are required on both sides of the target area, i.e., the slotted diaphragm.
DE 37 43 073 A1 discloses a balance, in which the transmitter and the receiver of the optical position sensor are accommodated inside the magnet cover. Furthermore, U.S. Pat. No. 3,805,907 discloses a null-type indicator for a balance, in which for purposes of adjustment the light sensitive areas can be pivoted about the optical axis of the receiver.
For reasons relating to mechanical precision and repeatability, weighing systems of this type are manufactured, as far as possible, in a monolithic manner. That is, they are machined from a uniform block of material, such as aluminum. However, in the aforementioned weighing systems known from the prior art, the pedestals for mounting the position sensor can be manufactured separately and then subsequently screwed to the base. For reasons relating to precision, in particular the position sensor has to be positioned very close to the target area. However, the resulting spatial constraint does not allow the slotted diaphragm of the target area to be cut out with the milling and drilling tools that are used to make the rest of the monolith. On the other hand, however, the monolithic attachment of the position sensor pedestals would be especially important with respect to the primary goal of increasing the resolution and repeatability in the weighing area. Due to the limited repeatability of the position of the position sensor relative to the slotted diaphragm that can be achieved with existing manufacturing methods, the net result is, in particular, assembly inaccuracies and effects, such as temperature effects on the threaded connections.